Method for magnetically developing electrostatic images

ABSTRACT

AN IMPROVED METHOD FOR MAGNETICALLY DEVELOPING ELECTROSTATIC LATENT IMAGES ON A PHOTOCONDUCTIVE SURFACE IN THE FORM OF A BELT. THE METHOD STEPS INCLUDE FORMING THE LATENT ELECTROSTATIC IMAGES ON A BELT SURFACE AND TRANSPORTING THE BELT SURFACE ALONG A PREDETERMINED PATH WHICH IS SUBSTANTIALLY PARALLEL TO AND CLOSELY SPACED TO AN ENDLESS BELT CARRYING DEVELOPER MATERIAL INCLUDING A MAGNETIC COMPONENT AND AN ELECTROSTATIC COMPONENT. THE PHOTOCONDUCTIVE BELT AND DEVELOPER MATERIAL TRANSPORT MOVE IN THE SAME DIRECTION. ONE OR MORE MAGNETIC FIELDS ARE ROTATED IN THE DEVELOPMENT ZONE ON AN AXIS WHICH MOVES IN THE SAME DIRECTION AS DEVELOPER MATERIAL TRANSPORT AND PHOTOCONDUCTIVE BELT. THE ROTATING MAGNETIC FIELDS MOVE AT SPEEDS WHICH ARE APPROXIMATELY TWO TO THREE TIMES THE SPEED OF THE DEVELOPER MATERIAL TRANSPORT AND ABOUT FOUR TO IVE TIMES THE SPEED OF HOTOCONDUCTIVE BELT SUCH THAT THE DEVELOPER MATERIAL IS CAUSED TO RAISED TOWARDS AND AWAY FROM THE ELECTROSTATIC IMAGES TO BE DEVELOPED IN A WAVE-LIKE FORMATION REPEATEDLY DURING THE DEVELOPMENT TO EFFFECT A HIGH QUALITY OF THE IMAGES. UPON DEVELOPMENT OF THE IMAGE THE DEVELOPED IMAGE IS TRANSFERRED TO A SUPPORT SHEET TO FORM A PERMANENT COPY. TO ENCHANCE SOLID AREA DEVELOPMENT AN ELECTRIC BIAS IS COUPLED TO THE DEVELOPER TRANSPORT TO SUPPRESS IMAGE BACKGROUND FIELD.

June 26, 1 973 PAUL s. I... WU 3,741,790

METHOD FOR MAGNETICALLY DEVELOPING ELECTROSTATIC IMAGES Original Filed May 28, 1968 PAUL S.L.WU BY AMA/1A A TWP/V5 Y United States Patent Oflicc 3,741,790 Patented June 26, 1973 US. Cl. 117-175 1 Claim ABSTRACT OF THE DISCLOSURE An improved method for magnetically developing electrostatic latent images on a photoconductive surface in the form of a belt. The method steps include forming the latent electrostatic images on a belt surface and transporting the belt surface along a predetermined path which is substantially parallel to and closely spaced to an endless belt carrying developer material including a magnetic component and an electrostatic component. The photoconductive belt and developer material transport move in the same direction. One or more magnetic fields are rotated in the development zone on an axis which moves in the same direction as developer material transport and photoconductive belt. The rotating magnetic fields move at speeds which are approximately two to three times the speed of the developer material transport and about four to five times the speed of photoconductive belt such that the developer material is caused to raise towards and away from the electrostatic images to be developed in a wave-like formation repeatedly during the development to effect a high quality of the images. Upon development of the image the developed image is transferred to a support sheet to form a permanent copy. To enhance solid area development an electric bias is coupled to the developer transport to suppress image background field.

This application is a divisional application of Ser. No. 732,737, filed May 28, 1968, now US. Pat. 3,592,166.

This invention relates to electrostatographic copying and, particularly, to an improved method for the deposition of visible powder material on an electrostatic latent image as in the development of a xerographic image or the like.

In xerography, it is usual to form an electrostatic image on a sensitized surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation as set forth, for instance, in US. Pat. 2,297,691 to Chester F. Carlson. Whether formed by this means or any other, the resulting electrostatic charge pattern is conventionally developed by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic image.

A common process of applying the developer to the electrostatic image described in US. Pat. 2,618,552 to E. N. Wise involves cascading a finely-divided colored material called a toner deposited on a slightly more coarsely divided material called a carrier across the electrostatic latent image areas.- The toner and carrier being rubbed against each other while cascading, impart an electrostatic charge to each other by triboelectric charging. When a carrier particle, bearing on its surface oppositely charged particles of toner, moves across an area on the image surface having an electrostatic charge, the charge on the surface exerts greater attraction for the toner than does the carrier and thus retains the toner in the charged areas and separates it from the carrier particle. The carrier particles, having greater momentum, will not be retained by the charged areas of the plate. When a toned carrier particle passes over a non-charged area of the plate, the electrostatic attraction of the carrier particles for the toner particles is suflicient to retain the toner on the carrier, preventing deposition in such areas as the carrier particles momentum carries both toner and carrier past.

The above process, referred to as cascade carrier developmen has a high development latitude and is particularly noteworthy in freedom from background deposition. Further, the process is dependable, operates with high efficiency under extreme humidity conditions and is easily converted to give either positive or reverse reproduction of the original to be copied. The process, however, has certain limitations. It gives little or no solid area coverage, that is, solid colored areas such as those presented by block letters develop only around the periphery leaving a white or undeveloped area in the center. Again, relying largely on gravity to move the carrier across the image-bearing surface, the process requires relatively large carrier particle sizes for best efficiency. As a result, using cascade development at high speeds places undue frictional stress upon the photoconductor surface and the developing materials as well as the equipment necessary to produce cascade movement of development material. In other words, at high speeds, the use of two-component developer material requires low impact of developing materials on photoreceptors and tightly sealed developer housings in order to prevent scattering and loss of toner particles and the usual carrier beads. Then, too, there is a tendency for smaller carrier particles to be retained on the plate thereby interfering with transfer of the toner image.

Closely related to the cascade carrier development is magnetic brush development as disclosed in US. Pat. 2,832,311. In this process a granular carrier is selected having ferromagnetic properties and being relative to the toner in a triboelectric series to impart the desired electrostatic polarity to the toner and carrier as in cascade carrier development. On inserting a magnet into such a mixture of toner and magnetic granular material, the carrier particles align themselves along the lines of force of the magnet to assume a brush-like array. The toner particles are electrostatically coated on the surface of the granular magnetic carrier particles. Development proceeds as in regular cascade carrier development on moving the magnet over the surface bearing the electrostatic image so that the bristles of the magnetic brush contact the electrostatic image-bearing surface.

Magnetic carrier development gives good coverage of solid areas and is eminently suitable for machine application by reason of the greater compactness of the developer system and freedom from dependence on gravity which limits the placement of a cascade carrier system around a rotary drum. Against these advantages, magnetic development is inherently less eflicient than cascade development. In magnetic development only part of the brush contacts the image-bearing surface. In addition, the magnetic field restricts the motion of the carrier particles interfering with the individual toner particles smoothly rolling across the image surface. As one consequence of this, a higher concentration of toner is generally essential in magnetic carrier development. By reason of this and the electrical characteristics which result in solid area coverage, the process gives a high background deposition and is generally characterized by poor development latitude.

It is therefore an object of this invention to improve the development of electrostatic latent images.

It is another object of the invention to provide method for applying electroscopic developer powder to an electrostatic latent image combining the advantages of both cascade and magnetic carrier development while minimizing limitations normally associated with each.

It is another object of the invention to enable high image quality at very high development speeds.

It is a further object of the invention to maintain solid area image quality with a minimum of background.

It is still a further object of the invention to produce very good solid area images while at the same time accomplishing very good line copy images at very high photoreceptor speeds using a minimum of developer materials.

It is a still further object of the invention to present developer particles to a relatively large area development zone in a manner more simple than used heretofore.

These and other objects of the invention are attained by presenting developer material comprising magnetic carrier and electroscopic developer powder particles in a continuous wave-like pattern across the electrostatic latent image to be developed. To accomplish this one or more rotating magnetic flux fields are utilized for imparting a fluidized fiow to developer material moved on a transport past the image to achieve high quality development for both line and solid area images.

A preferred form of the invention is shown in the accompanying drawings, wherein:

FIG. 1 is a schematic sectional view of a typical xerographic reproduction machine embodying the principles er the invention;

FIG. 2 is an isometric view partly broken away of the ievelopment apparatus;

FIG. 3 is a partial sectional view illustrating details of one aspect of the apparatus, and;

FIG. 4 is an enlarged view of a circled portion of FIG. 1 illustrating in greater detail the path of movement of the developer material across the image being developed.

For a general understanding of an electrostatographic copying system incorporating the present invention, reference is made to FIG. 1 in which various components of a typical system are schematically illustrated. As is usual in electrostatic copying, a light image of an original to be reproduced is projected onto the sensitized surface of a photoreceptor to form an electrostatic latent image thereon. Thereafter, the latent image is developed with the same or an oppositely charged developing toner material, depending upon negative-to-positive or positive-to-positive mode of reproduction, to form a xerographic powder image corresponding to the latent image on the surface. The powder image is then electrostatically transferred to a support surface such as a sheet of paper or the like to which it may be fixed by a suitable fusing device whereby the powder image is caused permanently to adhere to the support surface.

For purposes of the present disclosure, the xerographic reproduction machine includes an exposure station at which a light or radiation pattern of a document to be reproduced is projected by a lens 11 onto an electrostatographic surface which in this case is in the form of a flexible belt 12 supported on rollers 13 mounted in suitable bearings in the frame of the machine, one of which is driven in a counterclockwise direction by a motor (not shown) at a constant rate that is proportional to the scan rate for the document being reproduced whereby the peripheral rate of the belt surface is identical to the rate of movement of the projected light image of the document. Alternatively, a full frame exposure may be utilized with flash lamps as is known by those skilled in the art. The belt surface comprises a layer of photoconductive material, such as, vitreous selenium, on a conductive backing that is sensitized prior to exposure by means of a corona generating device 14.

The exposure of belt 13 to the document light image discharges the photoconductive layer in the areas struck .by light, whereby there remains on the belt surface an electrostatic latent image in configuration corresponding to the light image projected from the document. As the belt surface continues, the electrostatic latent image passes through a developing station in which there is positioned a developer apparatus 16 in accordance with the present invention as will be described hereinafter.

Positioned next and adjacent to the developing station is the image transfer station which includes a pair of rollers 18 for holding a support material in the form of paper web P against the surface of the belt to receive the developed xerographic powder image therefrom. The web P is moved in synchronism with the movement of the belt by means of a take-up roll 20 which drives the support material P from a supply roll 22. In order to insure identical movement of belt 12 and web P, a suitable pro gramming device may be utilized to effect continuous synchronous movement of these surfaces.

The transfer of the developed image from the belt surface to the transfer material is effected by means of a corona transfer device 23 that is located at place of contact between the transfer material and the belt. The corona transfer device 23 is similar to the corona discharge device 14 in that it includes an array of one or more corona discharge electrodes that are energized from a suitable high potential source and extend transversely across the belt surface and are substantially enclosed within a shielding member.

In operation, the electrostatic field created by the corona discharge device 23 of appropriate polarity is effective to attract the developer particles comprising the developed image from the belt surface and cause them to adhere electrostatically to the surface of the transfer material.

Immediately subsequent to the image transfer station, the transfer material is carried to a fixing device in the form of a heat type fuser assembly 25 whereby the developed and transferred xerographic powder image on the sheet material P is permanently fixed thereto. After fusing, the finished copy is preferably discharged. from the apparatus at a suitable point for collection externally of the apparatus.

The next and fi'final station in the device is a belt cleaning station having positioned therein a corona precleaning device 26 similar to the corona charging device 14 of appropriate polarity, negative for positiveto-positive mode of reproduction and positive for negative-to-positive mode of reproduction, to impose an electrostatic charge of the belt surface and residual powder image adherent thereto to aid in effecting removal of the powder. A belt cleaning device 27 in the form of a rotary brush 27 is adapted to remove any powder remaining on the belt.

In general the electrostatic charging of the xerographic belt in preparation for the exposure step and the electrostatic charging of the support surface to effect toned image transfer are accomplished by means of corona generating devices whereby electrostatic charge on the order of from 700 to 1000 volts is measured on the respective surface in each instance. Although any one of a number of types of corona generating devices may be used, a corona charging device of the type disclosed in Vyverberg Pat. No. 2,836,725 is used for both the corona charging device 14 and the corona transfer device 23, each of which is secured to suitable frame elements of the apparatus and connected to a suitable electrical circuit.

Referring now to FIGS. 2-4, there is shown in greater detail the development apparatus 16 according to the present invention. Development apparatus 16 comprises a frame 50 on which there is formed a trough 51 for containing a supply of developer material 53. Developer material 53 comprises a mixture of any suitable magnetic carrier particles and an electroscopic toner powder with the toner powder adhering the carrier particles as a result of triboelectric effect. Typical carrier materials comprise powdered iron including the types known commercially as alcoholized iron and carboxal iron, alloys of magnetic iron, such as, nickel-iron alloys, nickel-cobalt-iron alloys, and magnetic oxides, such as, hematite (Fe O and magnetite (Fe O and ferromagnetic ferrites. Where the ferromagnetic material does not have the desired triboelectric relationship to the toner, the ferromagnetic material may be used as a core and covered with a resinous coating having the desired triboelectric properties as described, for example, in US. Pat. 2,618,551 to Walkup and US. Pat. 2,874,063 to Greig. Typical electroscopic toner powders are described in US. Pat. 2,618,551 to Walkup, US. Pat. 2,618,552 to Wise, and U.S. Pat. 2,638,416 to Walkup and Wise.

The developer material is transported to the vicinity of the latent image at the development zone on a transport belt or web 55 supported at its ends by a pair of spaced apart cylinders or tubes 57 and 59 extending along parallel axes. Belt 55 is made out of any suitable flexible, electrically conductive material, such as, a conductive rubber, while cylinders 57, 59 are made of any suitable non-magnetic material for a reason which will become apparent. Belt 55 is desirably continuously driven through a drive pulley 61 which is drivingly connected to an extension 63 of a torque transmitting member 65 which in turn is drivingly connected to cylinder 59. Torque member 65 is rotatably supported in frame 50 as by a bearing 67 enabling cylinder 59 to turn in the frame. Cylinder 59 is rotatably supported at the opposite end from torque member 65 in a manner as will be described. It will be appreciated that cylinder 57, which is also rotatably supported in frame 50, turns due to movement imparted to belt 55 from cylinder 59.

To load the developer material 53 onto belt 55 there is mounted within cylinder 57 a fixed permanent bar magnet 71 on a concentric axis therewith. Bar magnet 71 is oriented in a polar path at approximately 45 degrees to the vertical axis of the magnet as indicated by letters N and S in FIG. 2 showing north and south poles, respectively. The magnet 71 is fixed in this position by any suitable means as by a key 73 received by frame 50 and a keyway 75 formed in a stub shaft 77 extending from magnet 71. Stub shaft 77 serves as a support for an end cap 81 of cylinder 57 which is rotatably supported on a bearing 83. The opposite end of magnet 71 and cylinder 57 is arranged in similar manner except that a key for maintaining the magnet in a fixed position is not necessary. It will be appreciated that the path of magnetic flux emanating from the top of magnet 71 as determined by the orientation of the poles returns through the air with the magnetic developer material 53 following these lines of magnetic flux. Thus, as the belt moves, a portion of developer material is continuously removed from the trough 51 onto the belt for transport through the development zone.

Positioned at the opposite belt end on a concentric axis with cylinder 59 is another fixed permanent bar magnet 85 which serves to retain developer material 53 on belt 55 as it leaves the development zone and returns to the trough. It should be noted that the pole orientation for bar magnet 85 is horizontal to effect optimum holding force at the belt end so that the developer material is returned to the trough with a minimum of spilling action. Also the magnetic flux lines running through the adjacent trough area cause good mixing of the developer material in this vicinity thereby enabling optimum retoning of the developer material. Bar magnet 85 is fixed in a manner similar to magnet 71. At one end a stub shaft 87 is formed with a keyway 89 which receives a key 91 also received in frame 50. A stub shaft 93 at the opposite end is supported by a bearing 95 received in torque member 65. An end cap 97 which is received in cylinder 59 is rotatably supported on stub shaft 87 by a bearing 99 thereby enabling the cylinder 59 to turn while magnet 85 remains fixed.

In accordance with the invention, developer material is moved past the latent electrostatic image in a continuous wave-like motion to achieve both a cascading effect of the material across the image as well as improved carrier and toner mixing, thereby resulting in high quality solid area and line development. To this end, one or more rotatable bar magnets 101 are disposed centrally on the interior of belt'55 adjacent to the development zone. It will now be appreciated that the rotating magnetic field causes the developer material which already has a horizontal velocity to rise into very close proximity with the latent image and then fall again due to the changing field. The path of the development particles within the development zone may be likened to that of a fluidized wave as best shown in FIG. 4 wherein two distinct particle paths X and Y are illustrated. This fluidized wave enables more intimate contact of the developer material with the image resulting in a greater proportion of the toner being in contact with the image. At the same time, the magnetic flux lines from the magnet 101 are suificiently strong to keep the smaller carrier particles from being retained on the image or from-otherwise sticking to it. The higher concentration of toner coming into contact with the image results in an improved solid area coverage as compared with the other known development techniques. In order to further enhance image development, a DC. source of potential 102 is appplied to conductive belt 55 to suppress image background fields thereby enabling background deposition to be minimized resulting in high quality prints. It has been found that voltages ranging from about volts to about volts are very good for this purpose.

Magnet 101 may be driven in any suitable manner as by a pulley 105 which drives a shaft 107 to which the magnet is secured, it being understood that pulley 105 is driven from any suitable power source. The rate at which the magnet 101 rotates should be such that it is sufficiently low so that the developer particles are able to respond and sufliciently high to obtain the desired wave pattern. It has been found that very good development results when magnet 101 rotates in the same direction as transport belt 55 and photoreceptor belt 12 in linear speeds from about 2 to about 3 times the transport belt speed and dfrom about 4 to about 5 times the photoreceptor spec As can be readily appreciated a certain amount of toner in the developer material picked up from the trough which is at a level slightly higher than the outside periphery of the belt is consumed during development. Any suitable means may be used for periodically replenishing the trough with a new supply of toner.

Above is described a novel method for the development of electrostatic latent images which produces a higher quality print than the brush-like streamers formed for ordinary magnet carrier development. With the apparatus of the invention there is a development characterized by cascade carrier development as well as development characterized by magnetic carrier development to give the combined advantages of each without the limitations ordinarily associated with these types of development. Moreover, due to the unique action of the particles on the image a much greater density is obtained for solid area coverage with a minimum of background deposition. It is quite apparent that such a development is greatly desired and has many advantages over the existing techniques.

While the present invention as to its objects and advantages has been described herein as carried out in a specific embodiment, it is not desired to be limited thereby; but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A method for electrostatically reproducing graphic information comprising forming a latent electrostatic image of the graphic information to be reproduced on an endless photoreceptor belt, transporting a development material including a magnetic component and an electrostatic component along a predetermined path substantially parallel and closely spaced to the latent image to be developed and in the same direction thereof, and creating magnetic fields moving about an axis of rotation and in the same direction as the development material at a speed about four to five times the speed of the photoreceptor belt and a speed two to three times the speed of the developer material to cause the development material to raise in close proximity with the latent image and then lower in wave-like formation repeatedly during development, and transferring the developed image to a support sheet.

References Cited UNITED STATES PATENTS 2,910,963 11/1959 Herman 117-17.5 2,930,351 3/1960 Giaimo 117-17.5

Hojo et a1. 117-17.5 Drexler 118-637 Flint 118-637 Knechtel 118-637 Kushirna 118-637 Renter 117-17.5 Clark et a1. 118-637 Sih 118-637 Lehmann 118-637 Thompson 101-114 US. Cl. X.R. 

